Reduced cost decoder using bitstream editing for image cropping

ABSTRACT

A method for decoding variable length encoded digital video data including pictures, to a size less than the full size of the pictures, each picture including a plurality of macroblocks. The method comprises the steps of receiving digital video data; parsing the digital video data to identify macroblocks included in the digital video data; discarding from the digital video data those macroblocks not associated with a picture region substantially corresponding to one of a safe-title picture region and safe-action picture region; and storing the digital video data in a decoder input buffer.

The present invention relates to video decoders in general and, moreparticularly, the invention relates to methods and apparatus forimplementing reduced-cost video decoders in a high definition orstandard definition television system.

BACKGROUND OF THE DISCLOSURE

Future digital television (DTV) receivers are expected to be implementedsubstantially in accordance with the transmission standards establishedby the Advanced Television Standards Committee (ATSC). A similarstandard is the European Digital Video Broadcasting (DVB) standard. Acompressed digital video system is described in the ATSC digitaltelevision standard document A/53, incorporated herein by reference.Moreover, the Moving Pictures Experts Group (MPEG) has promulgatedseveral standards relating to digital data delivery systems. The first,known as MPEG-1, refers to ISO/IEC standards 11172 and is incorporatedherein by reference. The second, known as MPEG-2, refers to ISO/IECstandards 13818 and is incorporated herein by reference.

The new DTV standards allow broadcasters to deliver virtually any formatup to 1920×1080 pixels. Specifically, DTV receivers must be capable ofreceiving source video comprising image sequences that vary in spatialresolution (480 lines, 720 lines, or 1080 lines), in temporal resolution(60 fps, 30 fps, or 24 fps), and in scanning format (2:1 interlaced orprogressive scan). Thus, the new DTV standards support either highdefinition television (“HDTV”), wherein the video frames are of higherresolution than those used in present NTSC signals, or standarddefinition television (“SDTV”), e.g., television which has approximatelythe same resolution per frame as the existing analog NTSC standard.

Because of the relatively large amount of data required to representeach frame of a HDTV picture, HDTV decoders must support much higherdata rates than SDTV decoders. The additional memory required by a HDTVdecoder, as compared to a standard SDTV decoder, and the increasedcomplexity of various circuitry within a HDTV decoder can make a HDTVdecoder considerably more expensive than an SDTV decoder.

One prior art technique for reducing memory requirements in a HDTVdecoder is identically disclosed in a commonly assigned set of threeU.S. patents, namely: U.S. Pat. No. 5,614,952, entitled DIGITAL VIDEODECODER FOR DECODING DIGITAL HIGH DEFINITION AND/OR DIGITAL STANDARDDEFINITION TELEVISION SIGNALS, issued Mar. 25, 1997; U.S. Pat. No.5,635,985, entitled LOW COST JOINT HD/SD TELEVISION DECODER METHODS ANDAPPARATUS, issued Jun. 3, 1987; and U.S. Pat. No. 5,614,957, entitledDIGITAL PICTURE-IN-PICTURE DECODER, issued Mar. 25, 1997. The abovethree patents [hereinafter the Boyce patents] are herein incorporated byreference in their entirety.

The Boyce patents disclose a series of techniques for reducing theamount of memory required to decode a bitstream including variablelength encoded video data. Essentially, high resolution images aredecoded at a reduced resolution, thereby requiring less memory to storethe images. Moreover, Huffman codes which represent higher-order DCTcoefficients are removed form the video stream. Unfortunately, while theBoyce techniques do reduce the total memory and processing requirementsof a decoder, the resulting decoder is still quite complex and costly.

Parts of an image to be displayed on a display device, i.e., thoseregions near the edges of the image, commonly contain information thatis not required for enjoyment of the image sequence. This is because ofthe uncertainty that these portions near the edges will be displayed atall. Such uncertainty is caused by two practices used by many televisionmanufacturers. First, manufacturers may include some amount of“overscan” in their display devices. Second, manufacturers may obscureportions of the picture area near the edges by the use of a displaybezel. In recognition of these (and other) practices, the Society ofMotion Picture Television Engineers (SMPTE) has adopted a RecommendedPractice (RP) 56-1990 which defines a “safe-action” area as thecenter-most 90% of the image, and a “safe-title” area as the center-most80% of the image. These portions are linearly determined (i.e., 90% and80% of vertical and horizontal dimensions).

In view of the above-described SMPTE standard and the need todramatically reduce system costs in DTV receivers (especially thoseassociated with, e.g., a small display screen), it is seen to bedesirable to reduce the video information within an encoded bitstream tocorrespond to the SMPTE “safe-title” or “safe-action” display sizes.

SUMMARY OF THE INVENTION

The invention relates to video decoders in general and, moreparticularly, the invention relates to methods and apparatus forimplementing reduced-cost video decoders in a high definition orstandard definition television system.

Specifically, the invention comprises a method for decoding variablelength encoded digital video data including pictures, to a size lessthan the full size of the pictures, each picture including a pluralityof macroblocks. The method comprises the steps of receiving digitalvideo data; parsing the digital video data to identify macroblocksincluded in the digital video data; discarding from the digital videodata those macroblocks not associated with a picture regionsubstantially corresponding to one of a safe-title picture region andsafe-action picture region; and storing the digital video data in adecoder input buffer.

BRIEF DESCRIPTION OF THE DRAWINGS

The teachings of the present invention can be readily understood byconsidering the following detailed description in conjunction with theaccompanying drawings, in which:

FIG. 1 depicts a high level block diagram of a video decoder accordingto the invention; and

FIG. 2 depicts a representation of a picture produced using the videodecoder of FIG. 1.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures.

DETAILED DESCRIPTION

The invention will be described within the context of a video decoder,illustratively an MPEG-2 video decoder, within a digital television(DTV) receiver, illustratively an ATSC television receiver. However, itwill be apparent to those skilled in the art that the invention isapplicable to any video processing system, including those systemsadapted to DVB, MPEG-1 and other information streams.

Referring now to FIG. 1, there is illustrated a video decoder generallyindicated by the reference number 100, implemented in accordance withone embodiment of the present invention. The illustrated decoder 100 iscapable of decoding HD and/or SD television signals, e.g., MPEGcompliant television signals. The video decoder 100 comprises abitstream trimmer 110, a buffer memory 120, a decoder 130, an outputbuffer 140 and an anchor frame memory 150. The decoder 130 comprises avariable length decoder (VLD) 131, an inverse quantizer (IQ) 122, aninverse discrete cosine transform (IDCT) circuit 133, a summer 134 and amotion compensation circuit 135.

Generally, the single most expensive element of a video decoder, interms of cost, is the anchor frame memory 150 which may comprise, e.g.,16 MB of synchronous random access memory (RAM) in a HD decoder. Theinput data buffer 120, which is used for the temporary storage of thecompressed bitstream represents a smaller, but not insignificant cost. Afully MPEG compliant HDTV decoder is expected to require at least 1 Mbof RAM for use as a data buffer.

Other elements of a decoder which add significantly to the cost of thedecoder are the inverse discrete cosine transform circuit 133 and theinverse quantizer circuit 132. The IDCT circuit 133 of a HDTV decoder isrequired to perform a large number of arithmetic computations at a highrate and, therefore, is likely to represent a significant portion of adecoder's circuitry. The IQ circuit 132 performs a smaller number ofcomputations than the IDCT circuit 133, but because of the high speedand complexity of the computations the cost of the IQ circuit 122 mayalso be a significant component of a HDTV decoder's overall cost. Inaddition, the motion compensation circuit 135 and variable lengthdecoder circuit 131 may require a significant amount of logic gates toimplement.

Because the cost and complexity of a HDTV decoder is largely a functionof the requirement that it process large amounts of data on a real timebasis, it is possible to reduce the complexity and thus the cost of aHDTV compatible decoder by reducing the amount of data that needs to beprocessed.

The previously mentioned Boyce patents disclose a series of techniquesfor reducing the amount of memory required to decode a bitstreamincluding variable length encoded video data. The Boyce patents disclosethe use of a preparser that reduces the data rate of an encoded videostream by limiting the number of DCT coefficients used to represent eachmacroblock (e.g., by discarding high-order DCT coefficients). Thereduced data rate bitstream is then decoded by a special decoder circuitto produce a reduced resolution video signal.

The bitstream trimmer 110 of the present invention does not selectivelyreduce the information included in each macroblock. Rather, thebitstream trimmer 110 identifies macroblocks that are not associatedwith a “safe-title” or, alternatively, “safe-action” portion of apicture per the SMPTE recommended practice 56-1990. The identifiedmacroblocks are discarded, and the resulting data-reduced bitstream isprocessed by the remaining circuitry within the decoder.

The visual effect of the bitstream trimmer 110 of the present inventionwill now be described with respect to FIG. 2, which depicts arepresentation of a picture produced using the video decoder of FIG. 1.Specifically, FIG. 2 depicts a picture 200 having a horizontal aspectdefined by X columns of macroblocks and a vertical aspect defined by Yrows of macroblocks. A first area 210 of picture 200 is the area definedby the SMPTE Recommended Practice 56-1990 as the “safe-title” area(i.e., the center-most 80% of the image). A second area 220 of picture200 is the area of the picture 200 that is outside of the aforementioned“safe-title” area. The second area 220 of picture 200 is not displayedon a display device because the macroblocks including image informationrepresenting this area are discarded by the bitstream trimmer prior tothe macroblock decoding circuitry.

The second area may be left as, e.g., a monochromatic border region. Inanother embodiment of the invention, the first area 210 represents the“safe-action” area (i.e., the center-most 90% of the image), while thesecond area 220 represents the undisplayed image area. In thisembodiment, fewer macroblocks are dropped and, therefore, more memoryand processing capability is required that in the case of discarding allbut the “safe-title” portion of the picture.

In the case of a 480 line by 704 pixel image generated using 16×16macroblocks, where each macroblock represents 256 luminance samples, 64Cb samples and 64 Cr samples, there are typically 44 columns and 30 rowsof macroblocks to provide a total of 1320 macroblocks. Thus, toapproximately remove all but the center-most 80% of a 480×704 pixelimage, the bitstream trimmer 110 removes the three top and bottom rowsof macroblocks (i.e., rows 1-3 and 28-30), and the four left and rightcolumns of macroblocks (i.e., columns 1-4 and 42-44).

The invention may, of course be practiced using formats other than the480×704 format discussed above. Table 1 lists several common videoformats, the number of macroblocks in each row and column of aparticular format, and the rows and columns to be discarded to reducethe picture size to the safe-area (i.e., 80%) picture size.

TABLE 1 Rows Of Deleted Macro- Columns Of Deleted Rows Columns ForFormat blocks Macroblocks For “Safe-Title” “Safe-Title” 1080 × 1920 68120 1-5, 64-68 1-3, 118-120  720 × 1280 45 80 1-3, 43-45 1-3, 78-80 480× 704 30 44 1-3, 42-44 1-4, 27-30

To identify macroblock rows and columns associated with the “non-safe”picture areas, the bitstream trimmer 110 operates in a variable lengthdecode mode to partially decode the input bitstream. Specifically, thebitstream trimmer 110 parses the picture_data( ) portion of thebitstream to, illustratively, retrieve the so-calledmacroblock_address_increment and responsively calculate the particularaddress for each examined macroblock. Those macroblocks having anaddress associated with the non-safe area are then discarded. it must benoted that the bitstream trimmer 110 does not decode the actual datawithin the macroblock. The bitstream trimmer 110 only examines thebitstream to identify various start codes and other codes syntacticallyassociated with the desired macroblock positional information.

In one embodiment of the invention, the bitstream trimmer 110 operatesin a fixed length mode to only discard rows of macroblocks. Since eachrow comprises one or more slices (where each slice comprises one or moremacroblocks), and the last slice in a row ends at the last column of arow, the bitstream trimmer 110 identifies a particular row byidentifying the slice(s) associated with the particular row. Eachmacroblock within the identified slice(s) is then discarded.Specifically, the bitstream trimmer 110 extracts the so-calledslice_vertical_position variable from the picture_data( ) portion of theinput bitstream S1. If the slice_vertical_position variable indicatesthat a particular slice is associated with a non-safe picture area, theparticular slice is discarded along with its macroblocks.

In another mode of operation, bitstream trimmer 110 receives a controlsignal MODE from a controller (not shown) that is indicative of a nativedisplay mode of a television receiver. That is, a television receivermay be receiving a high definition video signal, illustratively a1080×1920 signal, but may only be capable of displaying a standarddefinition picture, illustratively 480×640. Thus, in the case of adisplay device having a standard or low definition native operatingmode, the bitstream trimmer 110 is used to discard all macroblocksassociated with picture information outside of the native modecapabilities of the display device. In this case the memory andprocessing requirements are designed to support only the native mode ofthe display device.

In another mode of operation, the bitstream trimmer 110 receives a ratecontrol signal RATE from the input buffer memory 120. The optional ratecontrol signal provides an indication of the utilization level of thebuffer memory. To avoid buffer overflow and/or underflow conditions, thebitstream trimmer causes more or less data to be included in the datareduced bitstream S3 by, e.g., responsively discarding more or lessmacroblocks or by stripping away or adding padding information to thebitstream. The padding information may be specific, non-MPEG codes thatare used to increase the amount of data within the data reducedbitstream S3. if too much data is present in data reduced bitstream S3,the padding information is not inserted by the bitstream trimmer 110.

In still another embodiment of the invention, the input of input buffer120 is coupled to receive the input bitstream S1 directly, the input ofbitstream trimmer 110 is coupled to the output of input buffer 120, andthe output of bitstream trimmer 110 is coupled to the input of videodecoder 130. In this embodiment, since the bitstream trimmer 110 islocated after the input buffer 120, a normal (i.e., non-reduced) inputbuffer memory is required. However, this embodiment may be suitable forsystems in which a normal size input buffer 120 is required anyway(e.g., the output of the input buffer 120 is utilized by severaldecoders, some of which must have non-reduced bitstreams).

The above-described operation of bitstream trimmer 110 will result in areduced size picture, though the resolution of the displayed portion ofthe picture (i.e., pixel density) will remain unchanged. The operationof bitstream trimmer 110 will dynamically limit the amount of video datasupplied to the remaining elements of the video decoder circuit 100,including the VLD circuit 120, thereby reducing the amount of data thatmust be processed by the subsequent circuit elements on a real timebasis, and the required complexity of those circuit elements. Anadditional benefit of the use of the bitstream trimmer 110 is that itpermits for the use of a smaller input buffer 120 than would otherwisebe required.

Returning now to FIG. 1, the bitstream trimmer 110 receives a variablelength encoded bitstream S1 representing, e.g., a high definitiontelevision signal output from a transport demultiplexer/decoder circuit(not shown). The bitstream trimmer 110 parses the incoming bitstream S1,without performing a complete variable length decode operation, toidentify data corresponding to different types of video frames, such asbi-directionally coded (“B”) frames, predictively coded video frames(“P”) frames and intra-coded (“I”) frames. The bitstream trimmer 110then identifies and discards macroblocks within the identified framescorresponding to picture information outside of a preferenced picturearea, as previously described, to produce a reduced data bitstream S2that is coupled to the input buffer memory 120. The input buffer memory120 is used to temporarily store the variable length encoded data outputby the bitstream trimmer 110 until the variable length decoder 131 isready to accept the video data for processing. The VLD 131 has an inputcoupled to a data output of the input buffer memory 120 to retrieve thestored variable length encoded video data as data stream S3.

The VLD 131 decodes the retrieved data to produce a constant length bitstream S4 comprising quantized prediction error DCT coefficients, and amotion vector stream MV. The IQ circuit 132 performs an inversequantization operation upon stream S3 to produce a stream S5 comprisingquantized prediction error coefficients in standard form. The IDCTcircuit performs an inverse discrete cosine transform operation uponstream S5 to produce a stream S6 comprising pixel-by-pixel predictionerrors (degraded by quantization).

The summer 134 adds the pixel-by-pixel prediction error stream S6 to amotion compensated predicted pixel value stream S9 produced by themotion compensator 135. The output of summer 134 is a video stream S8comprising reconstructed pixel values (degraded by quantization) that iscoupled to the anchor frame memory 136 and to video processing circuitry(not shown) for further processing and/or display. The anchor framememory is accessed by the motion compensator 135 via signal path S10.The motion compensator utilizes one or more stored anchor frames (e.g.,the last frame of video produced at the output of the summer 134), andthe motion vector signal MV received from the VLD 131, to calculate thevalues for the motion compensated predicted pixel value stream S9.

By using the bitstream trimmer 110 in the above described manner, thecomputation requirements of the VLD circuit 120 are substantiallyreduced as compared to the case where all the received data is syntaxparsed and variable length decoded. The bitstream trimmer 110effectively limits the number of DCT coefficients which must be variablelength decoded by discarding macroblocks.

FIG. 3 depicts a representation of motion vector usage according to theinvention. Specifically, FIG. 3 depicts three related video frames(i.e., pictures), namely a first video frame 310, a second video frame320, and a third video frame 330. The frames are temporally related, inthat frames 310, 320 and 330 are displayed as respective first, secondand third frames within a sequence of video frames. The first videoframe 310 comprises a safe title area 312 and a non-displayed area 314.Similarly, the second video frame 320 comprises of a safe title area 322and a non-displayed area 324; and third video frame 330 comprises a safetitle area 332 and non-displayed area 334. Macroblocks within thenon-displayed area (i.e. areas 314, 324 and 334) are discarded aspreviously discussed.

FIG. 3 also depicts macroblocks 315, 325 and 335 within respective videoframes 310, 320 and 330. The three macroblocks represent a left to rightmotion of, e.g., an object within respective macroblocks 315, 325 and335. Macroblock 315 includes forward predictive information FP that isused to produce macroblock 325. Similarly, macroblock 335 includesbackward predictive information BP that is used to produce macroblock325 (assuming that video frame 320 is a B-frame).

Unfortunately, macroblock 315 is within the non-displayed area ofpicture 310 (i.e., macroblock 315 has been discarded prior to thedecoding of picture 310 and, therefore, the forward predictiveinformation FP within macroblock 315 has been lost. By contrast,macroblock 335 of picture 330 is within the displayed area 322, thus thebackward predictive information BP within macroblock 335 is available.Therefore, in the case of picture 320 being a B-Frame, macroblock 325only uses predictive information from macroblock 335.

In one embodiment of the invention, the case of picture 320 comprising aP-Frame is handled by simply ignoring the prediction information. Thatis, macroblocks that utilize predictive information derived fromnon-displayed (i.e., discarded) macroblocks are simply not updated withthis information. This may be accomplished by, e.g., setting thepredictive motion vectors associated with a discarded macroblock tozero.

In another embodiment of the invention, a sampling of the motion vectorprediction information utilized for macroblock surrounding a macroblock, i.e., macroblocks within the region of picture 320 denoted as326, are used to create an average prediction estimation which may beutilized to produce macroblock 325.

In still another embodiment of the invention, pixels that are normallypredicted using discarded macroblock information are simply notdisplayed (e.g., changed to black).

The preferred approach is the use of a regional prediction (i.e., anaverage of region 326 to predict macroblock 325). Thus, in the case offorward prediction information FP for macroblock 326 having beendiscarded, the macroblocks comprising a portion of the surroundingmacroblocks (e.g., region 326) of video frame 320 are sampled, and anaverage forward prediction parameter AFP is calculated. The AFP isutilized by the decoder to help construct macroblock 325. Similarly, inthe case of backward prediction information BP for macroblock 326 havingbeen discarded, the macroblocks comprising a portion of the surroundingmacroblocks (e.g., region 326) of video frame 320 are sampled, and anaverage backward prediction parameter ABP is calculated. The ABP isutilized by the decoder to help construct macroblock 325.

In another embodiment of the invention, the spatial displacement of thenon-discarded macroblock to the macroblock to be predicted is used todetermine an appropriate sampling region and sample weighting.Similarly, the spatial displacement of a region surrounding thenon-discarded macroblock may be compared to the spatial displacement ortemporal displacement) of a region surrounding the macroblock to bepredicted may be used to further refine the appropriate sampling regionand sample weighting.

Although various embodiments which incorporate the teachings of thepresent invention have been shown and described in detail herein, thoseskilled in the art can readily devise many other varied embodiments thatstill incorporate these teachings.

1. A method for decoding variable length encoded digital video dataincluding pictures, to a size less than the full size of the pictures,each picture including a plurality of macroblocks, the method comprisingthe steps of: parsing the digital video data to identify macroblocksincluded in the digital video data; discarding from the digital videodata those macroblocks not associated with a picture regionsubstantially corresponding to one of a safe-title picture region andsafe-action picture region; and storing the digital video data in adecoder input buffer, said digital video data stored in said decoderbuffer including fewer macroblocks per picture than identified duringsaid step of parsing.
 2. The method of claim 1, wherein: said step ofparsing comprises the step of extracting, from the digital video data,one of a macroblock address indicium and a slice position indicium; andsaid step of discarding comprises the step of discarding macroblocksassociated with a predetermined macroblock address indicium or apredetermined slice position indicium.
 3. The method of claim 1, furthercomprising the step of: identifying non-discarded macroblocks thatutilize predictive information associated with discarded macroblocks;and in the case of said identified non-discarded macroblocksadditionally utilizing predictive information associated withnon-discarded macroblocks, utilizing only said prediction informationassociated with non-discarded macroblocks to form said identifiednon-discarded macroblocks.
 4. The method of claim 3, wherein: in thecase of said identified non-discarded macroblocks utilizing onlypredictive information associated with non-discarded macroblocks,performing the steps of: identifying, for each identified non-discardedmacroblock, a respective proximately displayed macroblock region;utilizing, for each identified non-discarded macroblock, said respectiveproximately displayed macroblock region to estimate a respectiveregional motion compensation parameter; and calculating, for eachidentified non-discarded macroblock, a motion compensation parameterutilizing said respective regional motion compensation parameter.
 5. Themethod of claim 1, further comprising the step of: identifyingnon-discarded macroblocks that utilize predictive information associatedwith discarded macroblocks; identifying, for each identifiednon-discarded macroblock, a respective proximately displayed macroblockregion; utilizing, for each identified non-discarded macroblock, saidrespective proximately displayed macroblock region to estimate arespective regional motion compensation parameter; and calculating, foreach identified non-discarded macroblock, a motion compensationparameter utilizing said respective regional motion compensationparameter.
 6. An apparatus for use in a system for decoding variablelength encoded digital video data including pictures, to a size lessthan the full size of the pictures, each picture including a pluralityof macroblocks, the apparatus comprising: a bitstream trimmer, forparsing the digital video data to identify macroblocks included in thedigital video data, and for discarding from the digital video data thosemacroblocks not associated with a predefined picture region to producedata reduced encoded digital video data, said data reduced encodeddigital video data including fewer macroblocks per picture than presentin said digital video data; and a video decoder, coupled to saidbitstream trimmer, for decoding said data reduced encoded digital videodata to produce a video signal.
 7. The apparatus of claim 6, furthercomprising: a buffer memory, coupled to said bitstream trimmer and saidvideo decoder, for buffering said data reduced encoded digital videodata prior to said data reduced encoded digital video data being decodedby said decoder.
 8. The apparatus of claim 7, wherein said bitstreamtrimmer, in response to a rate control signal, responsively modifies adata rate parameter of said data reduced encoded digital video data. 9.The apparatus of claim 8, wherein rate control signal is generated bysaid buffer memory and is indicative of a level of memory utilizationwithin said buffer memory.
 10. The apparatus of claim 6, wherein saidbitstream trimmer, in response to a display mode indicium signal,responsively discards macroblocks associated with picture areas notrelevant to said indicated display mode.
 11. The apparatus of claim 6,wherein: said video decoder includes a motion compensator responsive tomotion predictive information included within said digital video data.12. The apparatus of claim 11, wherein: said motion compensator, in thecase of a macroblock utilizing motion predictive information from adiscarded macroblock, estimates said discarded motion compensationinformation.
 13. The apparatus of claim 12, wherein: said motioncompensator estimates said motion compensation information by utilizingan average motion estimation of a macroblock region proximate to saidmacroblock requiring said motion compensation information.
 14. Theapparatus of claim 13, wherein: said macroblock region comprises aregion calculated using at least one of a predetermined spatialdisplacement parameter, and a predetermined temporal parameter.
 15. Amethod for decoding variable length encoded digital video data includingpictures, to a size less than the full size of the pictures, eachpicture including a plurality of macroblocks, the method comprising thesteps of: parsing the digital video data to identify macroblocksincluded in the digital video data; discarding from the digital videodata only those macroblocks not associated with a picture regionsubstantially corresponding to one of a safe-title picture region andsafe-action picture region; and storing the digital video data in adecoder input buffer.
 16. The method of claim 15, wherein: said step ofparsing comprises the step of extracting, from the digital video data,one of a macroblock address indicium and a slice position indicium; andsaid step of discarding comprises the step of discarding macroblocksassociated with a predetermined macroblock address indicium or apredetermined slice position indicium.
 17. The method of claim 15,further comprising the step of: identifying non-discarded macroblocksthat utilize predictive information associated with discardedmacroblocks; and in the case of said identified non-discardedmacroblocks additionally utilizing predictive information associatedwith non-discarded macroblocks, utilizing only said predictioninformation associated with non-discarded macroblocks to form saididentified non-discarded macroblocks.
 18. The method of claim 17,wherein: in the case of said identified non-discarded macroblocksutilizing only predictive information associated with non-discardedmacroblocks, performing the steps of: identifying, for each identifiednon-discarded macroblock, a respective proximately displayed macroblockregion; utilizing, for each identified non-discarded macroblock, saidrespective proximately displayed macroblock region to estimate arespective regional motion compensation parameter; and calculating, foreach identified non-discarded macroblock, a motion compensationparameter utilizing said respective regional motion compensationparameter.
 19. The method of claim 15, further comprising the step of:identifying non-discarded macroblocks that utilize predictiveinformation associated with discarded macroblocks; identifying, for eachidentified non-discarded macroblock, a respective proximately displayedmacroblock region; utilizing, for each identified non-discardedmacroblock, said respective proximately displayed macroblock region toestimate a respective regional motion compensation parameter; andcalculating, for each identified non-discarded macroblock, a motioncompensation parameter utilizing said respective regional motioncompensation parameter.
 20. An apparatus for use in a system fordecoding variable length encoded digital video data including pictures,to a size less than the full size of the pictures, each pictureincluding a plurality of macroblocks, the apparatus comprising: abitstream trimmer, for parsing the digital video data to identifymacroblocks included in the digital video data, and for discarding fromthe digital video data only those macroblocks not associated with apredefined picture region to produce data reduced encoded digital videodata; and a video decoder, coupled to said bitstream trimmer, fordecoding said data reduced encoded digital video data to produce a videosignal.
 21. The apparatus of claim 20, further comprising: a buffermemory, coupled to said bitstream trimmer and said video decoder, forbuffering said data reduced encoded digital video data prior to saiddata reduced encoded digital video data being decoded by said decoder.22. The apparatus of claim 21, wherein said bitstream trimmer, inresponse to a rate control signal, responsively modifies a data rateparameter of said data reduced encoded digital video data.
 23. Theapparatus of claim 22, wherein rate control signal is generated by saidbuffer memory and is indicative of a level of memory utilization withinsaid buffer memory.
 24. The apparatus of claim 20, wherein saidbitstream trimmer, in response to a display mode indicium signal,responsively discards macroblocks associated with picture areas notrelevant to said indicated display mode.
 25. The apparatus of claim 20,wherein: said video decoder includes a motion compensator responsive tomotion predictive information included within said digital video data.26. The apparatus of claim 25, wherein: said motion compensator, in thecase of a macroblock utilizing motion predictive information from adiscarded macroblock, estimates said discarded motion compensationinformation.
 27. The apparatus of claim 26, wherein: said motioncompensator estimates said motion compensation information by utilizingan average motion estimation of a macroblock region proximate to saidmacroblock requiring said motion compensation information.
 28. Theapparatus of claim 27, wherein: said macroblock region comprises aregion calculated using at least one of a predetermined spatialdisplacement parameter, and a predetermined temporal parameter.